This invention relates to a method of co-producing dialkyl carbonate and alkanediol, and, in particular, to a method for enhancing the efficiency of the co-production by the use of zeolite supported alkali and/or alkaline earth metal present in excess of a stoichiometric amount.
Various homogeneous catalysts have been proposed for carbonate transesterification. For example, U.S. Pat. Nos. 3,642,858 and 4,181,676 disclose the preparation of dialkyl carbonates by tranesterifying alkylene carbonates with alcohols in the presence of alkali metals or alkali metal compounds without the use of a support material. U.S. Pat. No. 4,661,609 teaches the use of a catalyst selected from the group consisting of zirconium, titanium and tin oxides, salts or complexes thereof.
Commercial use of homogeneous catalysts is restricted because separation of the catalyst from the reactants can be difficult. Because the transesterification is an equilibrium reaction, in an attempt to isolate the intended dialkyl carbonate by distillation of the reaction liquid without advance separation of the catalyst, the equilibrium is broken during the distillation and a reverse reaction is induced. Thus, the dialkyl carbonate once formed reverts to alkylene carbonate. Furthermore, due to the presence of the homogenous catalyst, side reactions such as decomposition, polymerization, or the like concurrently take place during the distillation which decrease the efficiency.
Various heterogenous catalysts have also been proposed for carbonate transesterification. The use of alkaline earth metal halides is disclosed in U.S. Pat. No. 5,498,743. Knifton, et al., xe2x80x9cEthylene Glycol-Dimethyl Carbonate Cogeneration,xe2x80x9d J. Molec. Catal. 67:389-399 (1991) disclose the use of free organic phosphines or organic phosphines supported on partially cross-linked polystyrene. U.S. Pat. No. 4,691,041 discloses the use of organic ion exchange resins, alkali and alkaline earth silicates impregnated into silica, and certain ammonium exchanged zeolites. U.S. Pat. No. 5,430,170 discloses the use of a catalyst containing a rare earth metal oxide as the catalytically active component. The use of hydrotalcites is disclosed in Japanese patent application 3[1991]-44,354. Zeolites ion-exchanged with alkali metal and/or alkaline earth metal, thereby containing a stoichiometric amount of metal, are disclosed in U.S. Pat. No. 5,436,362.
Inorganic heterogenous catalysts generally possess thermal stability and easy regeneration. However, these catalysts, including the zeolites containing a stoichiometric amount of alkali or alkaline earth metal, generally demonstrate low activity and/or selectivity and are unsatisfactory for commercial application.
Polymer supported organic phosphines and ion exchange resins show high activity and good to excellent selectivity in transesterification reaction between alkylene carbonate and alkanol; however, these polymeric materials do not appear very stable and gradually lose catalytic activity over a long period of time, especially at relatively high temperatures.
Thus, there remains a need for a method of transesterifying alkylene carbonate with alkanol to co-produce dialkyl carbonate and alkanediol which will provide higher activity and selectivity over a wide temperature range.
A method is provided for co-producing dialkyl carbonate and alkanediol by reacting alkylene carbonate with alkanol in the presence of a zeolite catalyst which contains alkali metal, alkaline earth metal, or a combination thereof present in excess of a stoichiometric amount.
The preferred alkylene carbonate is ethylene carbonate and the preferred alkanol is methanol. Cesium is the preferred alkali metal.
The zeolite can be selected from the group consisting of ZSM-5, zeolite beta, ZSM-22, ZSM-23, ZSM-48, ZSM-35, ZSM-11, ZSM-12, Mordenite, Faujasite, Erionite, zeolite USY, MCM-22, MCM-49, MCM-56, and SAPO; ZSM-5 is most preferred.
Alkali metal, alkaline earth metal, or combination thereof can be incorporated into the zeolite by any known means which will allow at least a portion of the excess metal to occupy the zeolite pore space; such as by impregnation. In a preferred embodiment, alkali metal and/or alkaline earth metal within the zeolite pore is in an oxide form. For example, when cesium is used as the alkali metal, the excess cesium occupies the zeolite pore in the form of cesium oxide.
The process conditions include a reaction temperature of about 20xc2x0 C. (68xc2x0 F.) to about 300xc2x0 C. (572xc2x0 F.), a reaction pressure of about 14 to about 4000 psig, a liquid hour space velocity of about 0.1 to 40 hrxe2x88x921, and a molar ratio of alkanol to alkylene carbonate of about 1-20.
The transesterification catalysts of the current invention exhibit high activity and excellent selectivity in the reaction of alkylene carbonate with alkanol and are superior vs. zeolites in NH4+-form or containing stoichiometric amount of alkali and/or alkaline earth metal.
Unlike polymer catalysts such as ion exchange resins, the basic zeolite catalysts used in the method of the invention are thermally stable and regenerable. The combination of high catalytic activity and selectivity in a wide temperature range, and excellent thermal stability and regenerability of the catalyst render them suitable for commercial use in co-producing organic carbonate and alkanediol through ester exchange reaction.
The organic carbonates produced by the method of the invention, dimethyl carbonate in particular, have potential application as xe2x80x9cgreenxe2x80x9d replacements for phosgene that is used mainly in manufacture of polyurethane and polycarbonate resins.